Continuous bleaching of alkylpolyglycosides

ABSTRACT

A continuous method of bleaching an alkylpolyglycoside, substantially free of alcohol, with peroxy compounds, preferably hydrogen peroxide, which is highly efficient to provide an unexpected high degree of color reduction from a dark brown to a light, white product, from an extinction coefficient color respectively of about 10 to about 15 to about 0.025 to about 0.15. The bleaching is carried out at controlled pH and temperature, under pressure preferably in the presence of Mg or MgO.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an improved method for bleachingalkylpolyglycosides, and in particular to a continuous method ofbleaching with a peroxide, such as hydrogen peroxide.

2. Description of Related Art

Alkyl glycosides are conveniently prepared by reacting an alcohol of thetype and chain length which is desired to form the "alkyl" portion ofthe glycoside of interest with a saccharide reactant (e.g., amonosaccharide such as glucose, xylose, arabinose, galactose, fructose,etc., or a polysaccharide such as starch, hemicellulose, lactose,maltose, melibiose, etc.) or with a glycoside starting material whereinthe aglycone portion thereof is different from the alkyl substituentdesired for the ultimate alkyl glycoside product of interest. Typically,such reaction is conducted at an elevated temperature and in thepresence of an acid catalyst. Various alkyl glycoside products andprocesses for making same are disclosed in a variety of representativepatents. U.S. Pat. No. 4,987,225 contains an extensive listing ofprocesses for preparing alkyl glycoside compositions. As disclosedtherein, processes for preparing alkyl glycoside compositions aredisclosed in U.S. Pat. No. 3,219,656 to Boettner (issued Nov. 23, 1965);U.S. Pat. No. 3,547,828 to Mansfield et al. (issued Dec. 15, 1970); U.S.Pat. No. 3,598,865 to Lew (issued Aug. 10, 1971); U.S. Pat. No.3,707,535 to Lew (issued Dec. 26, 1972); U.S. Pat. No. 3,772,269 to Lew(issued Nov. 13, 1973); U.S. Pat. No. 3,839,318 to Mansfield (issuedOct. 1, 1974); U.S. Pat. No. 4,349,669 to Klahr (issued Sep. 14, 1982);U.S. Pat. No. 4,393,203 to Mao et al. (issued Jul. 12, 1983); U.S. Pat.No. 4,472,170 to Hellyer (issued Sep. 18, 1984); U.S. Pat. No. 4,510,306to Langdon (issued Apr. 9, 1985); U.S. Pat. No. 4,597,770 to Forand etal. (issued Jul. 1, 1986); U.S. Pat. No. 4,704,453 to Lorenz et al.(issued Nov. 3, 1987); U.S. Pat. No. 4,713,447 to Letton (issued Dec.15, 1987); published European Application No. 83302002.7 (EPOPublication No. 0092355; Vander Burgh et al; published Oct. 26, 1983);published European Application No. 83200771.0 (EPO Publication No.0096917; Farris; published Dec. 28, 1983); and published EuropeanApplication No. 84303874.6 (EPO Publication 0132043; published Jan. 23,1985). Other representative patents are U.S. Pat. No. 2,235,783 (White,issued Mar. 18, 1941); U.S. Pat. No. 2,356,565 (Chwala, issued Aug. 22,1944); U.S. Pat. No. 2,390,507 (Cantor, issued Dec. 11, 1945); U.S. Pat.No. 2,442,328 (Young, issued Jun. 17, 1947); U.S. Pat. No. 3,375,243(Nevin et al., issued Mar. 26, 1968); U.S. Pat. No. 3,450,690 (Gibbonset al., issued Jun. 17, 1969); U.S. Pat. No. 3,640,998 (Mansfield etal., issued Feb. 8, 1972); U.S. Pat. No. 3,721,633 (Ranauto, issued Mar.20, 1973); U.S. Pat. No. 3,737,426 (Throckmorton et al., issued Jun. 5,1973); U.S. Pat. No. 3,974,138 (Lew, issued Aug. 10, 1976); U.S. Pat.No. 4,011,389 (Langdon, issued Mar. 8, 1977); and U.S. Pat. No.4,223,129 (Roth et al., issued Sep. 16, 1980).

In the preparation of alkyl glycoside products, it is not uncommon forsuch products to develop an undesirably dark coloration during thecourse of the synthesis and isolation procedures employed. Variousprocedures have been suggested for improving the color of such darkcolored glycoside products including, for example, treatment withbleaching reagents such as hydrogen peroxide; intentional colorformation by heat treatment under alkaline conditions followed byremoval (e.g., by precipitation, filtration, etc.) of dark coloredimpurities generated during said treatment procedure; treatment withdecolorizing adsorbents such as particulate carbon materials, etc.; andthe like. See in this regard, for example, Gibbons' U.S. Pat. No.3,450,690 which discloses an alkaline heat treatment/separationprocedure that can optionally be followed by treatment with bleachingagents such as hydrogen peroxide or by treatment with decolorizingcarbons. See also Cantor's U.S. Pat. No. 2,390,507; White's U.S. Pat.No. 2,235,783; Example 1 of Throckmorton et al.'s U.S. Pat. No.3,737,426; Examples 5 and 10 of Langdon's U.S. Pat. No. 4,011,389; andExample 1 of U.S. Pat. No. 4,472,170 to Hellyer (issued Sep. 18, 1984)for teachings related to the use of carbon adsorbents for thedecolorization of various alkyl glycoside products.

Even when glycoside products are originally prepared (or aresubsequently decolorized in accordance with one or more of theprocedures set forth above) in a fashion which results in initial colorcharacteristics acceptable for certain applications, such productsnonetheless commonly exhibit a propensity to discolor (i.e., darken) asa function of time even under relatively mild storage conditions (e.g.,at neutral or slightly acidic pH and ambient conditions, i.e., 20°C.-35° C.). The propensity to discolor is greatly accentuated (i.e., interms of the intensity and rapidity thereof) by exposure to elevatedtemperatures (such as, for example, in the range of 35° C. to 100° C. ormore) and/or exposure to relatively strong alkaline aqueous environments(i.e., pH of 8 to 12). Generally speaking, the extent of discolorationis related to the severity of the pH/temperature/time to which theglycoside product is exposed. In U.S. Pat. No. 4,557,729 to McDaniel etal. (issued Dec. 10, 1985), the aforementioned problem of colordeterioration of glycoside products during storage thereof is discussedand a method for obviating such problem is disclosed which entails firstbleaching the glycoside product of interest with an oxidizing agent suchas ozone, hydrogen peroxide, hypochlorite, etc., and thereafter exposingthe resulting bleached glycoside product to a source of sulfur dioxide(e.g., sulfur dioxide gas, sodium sulfite, sodium metabisulfite, sodiumhydrosulfite, etc.) to stabilize said glycoside product against colordegradation. Another McDaniel et al. Patent, U.S. Pat. No. 4,904,774,notes the discoloration tendency of glycosides which have beendecolorized by bleaching with peroxide materials such as hydrogenperoxide, upon exposure to high temperatures, and proposes colorreduction by hydrogenation under catalytic hydrogenation conditionsusing materials such as Raney nickel or sodium borohydride. U.S. Pat.No. 4,990,605 to Lueders (issued Feb. 5, 1991) describes a method ofmanufacturing light colored alkyloligoglycosides by treatment withactivated carbon followed by distillation and bleaching with a peroxidecompound, preferably hydrogen peroxide, at temperatures of 50° to 100°C. under neutral or alkaline pH. Example 1, and comparative Example A,illustrates and compares the process with and without the activatedcarbon treatment.

The overall process to prepare light colored alkylpolyglycosidesurfactants accordingly typically involves reaction of an alcohol with asaccharide in the presence of an acid catalyst followed byneutralization of the acid catalyst, removal of the alcohol andbleaching of the resultant substantially alcohol-free alkylpolyglycosideproduct, followed usually by a stabilization treatment to provide colorstability. In the past, the process has been conducted as a batchprocess, and while it was recognized that a continuous process would bedesirable, no practical continuous process has been developed to providea very light colored alkylpolyglycoside surfactant product.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a diagrammatic flow chart of the overall process ofpreparation of a light color, stable, alkylpolyglycoside product showingthe reaction of the alcohol and saccharide in the presence of an acidcatalyst, followed by neutralization, removal of alcohol by evaporation,the bleaching process of the present invention and subsequentstabilization.

FIG. 2 of the drawing is a more detailed flow chart of the continuousbleaching step of the present invention to provide a light coloredalkylpolyglycoside surfactant.

BRIEF SUMMARY OF THE INVENTION

It has now been discovered that alkylpolyglycosides can be continuouslybleached in a well-mixed, pressurized, vessel by metering in a crudealkylpolyglycoside feed (preferably substantially alcohol-free),preferably hydrogen peroxide (as the peroxy bleaching agent), andcaustic and in the presence of more than about 250 ppm Mg (in the MgOform) per pound dry solids crude, into a bleaching matrix. The causticaddition is adjusted to maintain the reaction pH above 9, preferablyabove 10, by metering in from about 1.0 to about 1.2 moles of causticper mole of peroxide and about 0.25 to about 2% wt./wt. peroxide perpound of dry solids polyglycoside. The pressure of the reaction mixturebuilds due to liquid hydraulics, including the water vapor pressure andoxygen generated from degradation of hydrogen peroxide and is controlledat an elevated, economical level. Based on the vessel pressure control,the bleached product is vented off as a foamy liquid and subjected tofurther downstream processing, such as stabilization.

The continuous process of this invention avoids the disadvantages ofprevious approaches involving batch processing. The present processallows for bleaching at temperatures which minimize discoloration andhandling problems below about 120° C., preferably below 100° C. and mostpreferably in the range of about 90° to about 100° C., the nominaltemperature limit of atmospheric pressure bleaching due to foamexpansion and boiling. While the choice of operating temperature isprimarily an economical consideration, it must balance investmentcapital (smaller reactor, smaller agitator, higher pressure reactorsystem and more reactor heating/cooling equipment with highertemperature) versus operating costs (higher hydrogen peroxideconsumption, more product degradation, higher heating and cooling dutiesand less agitation power with higher temperature). In a production mode,where the product rate and equipment volume is fixed, the final productcolor is controlled by adjustment of the combination of the reactortemperature, reactor pressure and hydrogen peroxide dosage, which willbe discussed in more detail hereinafter.

The use of a continuous tank reactor with the bleaching matrix, whichincludes the crude feed, peroxide, caustic and the Mg, provide asignificant improvement in bleaching efficiency, improving the overallbleaching efficiency beyond that possible in a batch reaction. Anotheradvantage of this continuous process is that it provides a repeatableand less operationally intensive processing technique. Also, the use ofa pressure vessel provides the ability for bleaching to be conducted attemperatures greater than 100° C., thereby decreasing processing time byaccelerating bleach reaction kinetics. Furthermore, the use of apressure vessel lowers the volume of foam generated during thisbleaching process, thereby reducing the required size of the vessel andthe apparent viscosity of the foam. Reduction of the foam viscosityimproves vessel agitation efficiency and heat transfer of the foamyliquid.

DETAILED DESCRIPTION AND PREFERRED EMBODIMENTS

Other than in the operating examples, or where otherwise indicated, allnumbers expressing quantities, or reaction conditions used herein are tobe understood as modified in all instances by the term "about."

In view of the Summary above, it is accordingly an object of theinvention to provide an improved process for preparing light coloredalkylpolyglycosides of a saccharide reacted with an alcohol in thepresence of an acid catalyst at elevated temperatures, after which theacid catalyst is neutralized and the excess alcohol removed, and thesubstantially alcohol-free alkylpolyglycoside product is bleached andstabilized, in which the improvement comprises a controlled, continuous,bleaching step wherein said alkylpolyglycoside is bleached at analkaline pH and elevated temperature with a peroxy bleaching agentpreferably in the presence of magnesium in the form of the oxide, MgO.

As described in the related art section above, the initial reactionproduct of the alcohol and saccharide in the presence of an acidcatalyst results in a glycoside product. The product is a mixture of amonoglycoside of the alcohol and various higher degrees ofpolymerization (DP) polyglycosides in progressively decreasing molepercentage amounts, i.e., the diglycoside (DP2), the triglycoside (DP3)and the higher polyglycosides (DP4 and higher). The typical, statisticaldistribution of the various oligomers provided referred to as a Florydistribution. While the specific distribution of the various fractionsmay vary somewhat for various reaction products, the overalldistribution curve is the same, though the average DP of the reactionmixture may vary due to the differing distribution of the variousfractions, i.e., DP1, DP2, DP3 and higher fractions. Typically, theFlory distribution of the reaction product after removal of the excessalcohol will have an average degree of polymerization above 1.2, i.e.,about 1.4, with a monoglycoside content in the range of about 50-70% byweight of the glycoside product. Commercially available productstypically have an average Flory DP of about 1.3-1.7.

The glycoside products of the reaction of an alcohol and saccharide maybe represented by the formula I:

    ROG.sub.x                                                  (I)

wherein R is a residue of an alcohol, O is oxygen, G is a glycosideresidue, and x is the average degree of polymerization (DP) resultingfrom weighting of the various mono-, di-, tri- and higher glycosidefractions present in the product and is a number of from about one toabout three.

The average degree of polymerization is thus defined as the ratio ofsaccharide rings to the R groups in the alkyl glycoside. Themonoglycoside fraction would have one saccharide ring, the diglycosidewould have 2, the triglycoside would have 3 with the higher glycosidehaving corresponding more rings, the average of which in the currentlyavailable commercial product therefore being typically greater thanabout 1, generally in the order of about 1.2 to about 1.7, withpreferred mixtures at about 1.3 to about 1.7.

The alkyl polyglycoside products represented by the formula abovecontain a lipophilic group, the R group, and a hydrophilic group, theOG_(x) group. For detergent or surfactant-use application, the productshould have a hydrophilic-lipophilic balance (HLB) of from about 10 toabout 16, and preferably about 11 to about 14. The HLB value of aproduct may be calculated by the formula ##EQU1## where AGU is typicallythe anhydro glucose unit in G having a molecular weight of 162, MW_(o)is the molecular weight of oxygen and MW_(R) is the molecular weight ofthe R group, and DP is the average degree of polymerization as predictedby Flory's statistical treatment.

The lipophilic R groups in the alkyl polyglycosides are derived fromalcohols, preferably monohydric, for the detergent, surfactant-useapplications and should contain from about 8 to about 20, preferablyabout 9 to about 18 carbon atoms, with an average of about 10 to about13 being most preferred, to provide R groups of sufficient length fordetergent, surfactant-use applications. While the preferred R groups aresaturated aliphatic or alkyl, there may be present some unsaturatedaliphatic hydrocarbon groups. Thus, the preferred groups are derivedfrom the fatty alcohols derived from the naturally-occurring fats andoils, such as octyl, decyl, dodecyl, tetradecyl, hexadecyl, octadecyl,oleyl and linoleyl, but R groups may be derived from syntheticallyproduced Ziegler alcohols or oxo alcohols containing 9, 10, 11, 12, 13,14 or 15 carbon atoms. The alcohols of naturally-occurring fatty acidstypically contain an even number of carbon atoms and mixtures ofalcohols are commercially available such as mixtures of C₈ and C₁₀, C₁₂and C₁₄, and the like. Synthetically-produced alcohols, for examplethose produced by an oxo process contain both an odd and even number ofcarbon atoms such as the C₉, C₁₀, C₁₁ mixtures, which are also availablecommercially.

Glycoside products suitable for treatment in accordance with the presentinvention also include derivatives of products of the formula I aboveincluding, for example, those in which one or more of the normally free(i.e., unreacted) hydroxyl groups of the saccharide moiety, G, have beenalkoxylated, preferably, ethoxylated or propoxylated, so as to attachone or more pendant alkoxy or polyalkoxy groups in place thereof. Theformula (I) above, in order to encompass both alkoxylated andnon-ethoxylated products, may be modified to the formula II:

    RO (R.sup.1 O).sub.y G.sub.x                               (II)

where R, O, G and x are as defined earlier, R¹ is a divalent hydrocarbonradical of the alkoxylating agent, typically containing from 2 to about4 carbon atoms and y is a number having an average value of from 0 toabout 12, more preferably 0 to about 5. When y is 0, the formula reducesto formula I above and the product is non-alkoxylated.

Saccharide reactants which can be employed to prepare the aforementionedglycoside surfactants include reducing monosaccharide materialscontaining 5 or 6 carbon atoms such as, for example, glucose, galactose,mannose, xylose, arabinose, fructose, etc. as well as materials whichare hydrolyzable to form monosaccharides such as lower alkyl glycosides(e.g. methyl glycoside, ethyl glycoside, propyl glycoside, butylglycoside, etc.), oligosaccharides (e.g. sucrose, maltose, maltotriose,lactose, zylobiose, melibiose, cellobiose, raffinose, stachyose, etc.)and other polysaccharides. Such saccharide reactants may be employed indry (e.g. anhydrous) form or, if desired, may be employed in the form ofhydrated solids or aqueous solutions thereof. If utilized in the form ofa solution, it is preferred that the resulting reaction mixture containonly small amounts of water, i.e., less than about 1% by weight,preferably less than about 0.5% i.e. less than 0.25 or 0.1%.

While the preparation of the initial alkyl glycosides reaction mixtureemployed in the present invention forms no direct part of the presentinvention, a brief description generally of the preparation follows. Themolar ratio of alcohol to monosaccharide in the reaction mixture canvary widely but is typically between about 1.5:1 to about 10:1, andpreferably between about 2.0:1 to about 6.0:1. The particular molarratio chosen depends upon the desired average degree of polymerization(DP) of the monosaccharide reacted with the alcohol. Preferably, theratio of alcohol to monosaccharide will be chosen to allow theproduction of an alkyl glycoside product having a DP between about 1.2to about 1.7, and more preferably about 1.3 and about 1.6.

The reaction, as shown in FIG. 1, between the hydrophobic alcoholreactant and the saccharide reactant to form the glycoside surfactant istypically conducted at an elevated temperature and in the presence of anacid catalyst. As a general rule, said reaction is preferably conductedat a temperature of from about 80° to about 140° C., preferably about90° to about 120° C., and at pressures (about 10 to about 100 mm Hgabsolute), which facilitate water removal, while at the same timemaintaining the desired reaction temperatures.

Acid catalysts suitable for use include strong mineral acids such ashydrochloric acid, sulfuric acid, nitric acid, phosphoric acid,hypophosphorous acid, etc.; strong organic acids such as paratoluenesulfonic acid, methanesulfonic acid, triflouromethanesulfonicacid, mono- or polyalkylated aryl mono- or polysulfonic acids such asdodecylbenzenesulfonic acid, etc.; and macroreticular acidic ionexchange resins such as macroreticular sulfonic acid ion exchangeresins, perfluorinatedsulfonic acid resins, etc. Typically, said acidcatalyst will be employed in an amount ranging from about 0.0005 toabout 0.03 (preferably from about 0.002 to about 0.015) moles thereofper mole of saccharide used.

Typically, the above-described reaction process will be conducted over areaction period of from about 1 to about 20 (preferably from about 2 toabout 10) hours. Upon completion of the reaction, the acid catalyst istypically neutralized as indicated in FIG. 1 by an alkaline substance,preferably an alkali metal hydroxide such as sodium hydroxide, used inan amount about equal, on a stoichiometric basis, to the amount ofmaterial needed to neutralize the catalyst. For the present invention,most preferably the mixture is neutralized and adjusted to a pH in therange of about 9 to about 10 with an alkali metal hydroxide and alkalineearth metal oxide, such as magnesium oxide, prior to removal of thealcohol.

After neutralization of the acid catalyst, typically excess unreactedalcohol is removed. Alcohol removal is generally accomplished byevaporation, e.g. distillation, of the alcohol as indicated in FIG. 1.The use of a wiped film or thin film evaporator is particularlyconvenient for this purpose, preferably operated at about 150°-220° C.and about 0.1 to about 50 mm Hg pressure. More generally pressures ofabout 1 to about 100 mm Hg and temperatures of about 140° to about 230°C. may be employed. Other methods of removal of the alcohol may beemployed including distillation techniques and supercritical extractionunder conditions for removing alcohol to levels below about 5%, moredesirably below about 2% by weight to about 0.5%.

At this point, the resulting commercial product, substantially devoid ofalcohol, is typically a mixture of alkyl glycosides, in which forpurposes of this invention the average alkyl group will contain fromabout 8 to about 20, preferably about 9 to about 18, most preferably anaverage of about 10 to about 13, carbon atoms, having the typical Florydistribution discussed earlier above.

After removal of the excess alcohol to a level less than about 5% andpreferably less than about 1% by weight, the substantially alcohol-freeproduct is then bleached to a light color by the continuous bleachingprocess of the present invention. The present invention is alsoapplicable to glycoside products which have been molecularly distilledto further remove a portion of the monoglycoside present, particularlythose in which sufficient monoglycoside is removed to provide a mixtureof alkylpolyglycosides having varying degrees of polymerization of 2 andhigher in progressively decreasing amounts, in which the amount byweight of polyglycoside having a degree of polymerization of 2, ormixtures thereof with the polyglycoside having a degree ofpolymerization of 3, predominate in relation to the amount ofmonoglycoside, said composition having an average degree ofpolymerization of about 1.8 to about 3. Such compositions can beprepared by separation of the monoglycoside from the original reactionmixture of alkyl monoglycoside and alkyl polyglycosides after removal ofthe alcohol. This separation may be carried out by moleculardistillation which normally results in the removal of about 70-95% byweight of the alkyl monoglycosides. After removal of the alkylmonoglycosides, the relative distribution of the various components,mono- and poly-glycosides, in the resulting product changes and theconcentration in the product of the polyglycosides relative to themonoglycoside increases as well as the concentration of individualpolyglycosides to the total, i.e. DP2 and DP3 fractions in relation tothe sum of all DP fractions. Such compositions are disclosed in commonlyassigned copending application Ser. No. 07/810,588, filed on Dec. 19,1991, now U.S. Pat. No. 5,266,690, the entire contents of which areincorporated herein by reference. Molecular distillation as describedtherein is a short path, high vacuum distillation. On a laboratoryscale, pressures of about 0.1 mbar and lower may be employed. On acommercial scale, pressures will desirably be in the range of 0.01 mbarand lower, preferably about 0.001 mbar or lower. In the moleculardistillation, a very short distance is employed between the vaporizationand condensing surfaces, which is as close as possible. In practice theactual gap is bounded by about 0.1 to about 10 times the mean free pathof distilling molecules which is defined by kinetic theory. Theresidence time is as short as possible to minimize thermal degradationof the alkyl polyglycosides, less than about 2 minutes and preferablyless than about 15 seconds. With removal of at least about 50% of themonoglucosides, the molecularly distilled products will have an averageDP of at least about 1.8 and about at least 0.2 units higher than theaverage DP of the initial feed to the molecular still. If the amount ofmonoglycoside separated from the original reaction mixtures is in asufficient amount to provide that the monoglycoside retained in theresulting product is less than the total of DP2 and DP3 fractions, ormore preferably, less than that of the DP2 fraction, it can be readilyseen that a "peaked" distribution results i.e. that the DP2 and DP3distribution now illustrates a reduced or non-monoglycoside "peaked"distribution in the resulting products, which retain the DP4 and higherfractions in the resulting products prepared by molecular distillation.

The present continuous bleaching process is accordingly applicable toalkylpolyglycosides from which only the excess alcohol has been removedor those which have been treated so as to remove at least a portion,including removal of a significant portion of monoglycoside.

In still another embodiment, the continuous process of bleaching is alsoapplicable to compositions comprised of a mixture of two or more of atleast binary components of alkylpolyglycosides wherein each binarycomponent is present in the mixture in relation to its average carbonchain length in an amount effective to provide the surfactantcomposition with the average carbon chain length of about 9 to about 14and wherein at least one, or both binary components, comprise a Florydistribution of polyglycosides derived from an acid-catalyzed reactionof an alcohol containing 6-20 carbon atoms and a suitable saccharidefrom which excess alcohol has been separated. Such compositions aredisclosed in commonly assigned application Ser. No. 07/774,430, filed onOct. 10, 1991, now abandoned, the entire contents of which areincorporated herein by reference. As described therein, in commercialpractice alkylpolyglycosides are prepared from binary or ternarymixtures of alcohols providing the corresponding mixtures (binary orternary) of alkylpolyglycosides. The application accordingly describesthe preparation of alkylpolyglycoside compositions having a preselectedor predetermined average alkyl chain length and surfactant propertiesprepared from commercially available at least binary mixtures. Afterselecting the predetermined average carbon chain length of the alkylmoiety, the composition having the desired detergent or surfactantproperties is prepared by mixing two or more of at least binarycomponents, each binary component having an average alkyl chain lengthsuch that when mixed the amounts of the binary components are effectiveto provide the predetermined selected average alkyl moiety andsurfactant properties. Thus, the composition may contain a mixture of C₈-C₁₀, C₁₀ -C₁₂, C₁₂ -C₁₃, C₁₂ -C₁₆, C₁₂ -C₁₄, C₁₄ -C₁₅, C₁₆ -C₁₈, aswell as one containing C₉ -C₁₀ -C₁₁ and C₁₂ -C₁₄ -C₁₆alkylpolyglycosides and the like.

Thus, the bleaching process of the present invention, while preferablyuseful as a step in the overall preparation of alkylpolyglycosidesincluding the initial reaction, neutralization, alcohol removal,decolorization and stabilization (as shown in FIG. 1), is alsoapplicable to alkylpolyglycosides of (a) a single alkyl group, (b)commercially available mixtures of alkylpolyglycosides having two ormore different alkyl groups and (c) to molecularly distilled products inwhich the products have a peak of the DP2, or DP2 and DP3 componentstherein.

In its broadest embodiment, the present invention is a method ofreducing the color of an alkylpolyglycoside comprising the steps of

(a) providing an aqueous solution of the alkylpolyglycoside;

(b) continuously introducing said aqueous solution from step (a) to ableaching zone maintained at an elevated temperature suitable forbleaching;

(c) adjusting and maintaining the pH of the aqueous solution in saidbleaching zone at an alkaline pH;

(d) contacting the aqueous solution at the alkaline pH with a peroxybleaching agent in an amount effective to bleach and reduce the color ofthe alkylpolyglycoside; and

(e) continuously removing alkylpolyglycoside from said bleaching zonewherein the alkylpolyglycoside has a Klett color below about 50 and aresidual bleaching agent level below about 1000 ppm, preferably belowabout 500 ppm.

In the process above, the alkylpolyglycoside is one obtained from thereaction at elevated temperatures of an alcohol and saccharide in thepresence of an acid catalyst as discussed earlier above, afterneutralization of the acid catalyst and removal of substantially allexcess alcohol. In vessel 1 of FIG. 2, the alkylpolyglycoside is dilutedwith water to form an aqueous solution containing about 30 to about 85%by weight dry solids (ds) alkyl polyglycoside, preferably about 50 toabout 75% and most preferably about 53 or 55 to about 73%. Thealkylpolyglycoside product from the evaporation of the alcohol containsless than about 5% alcohol, preferably less than 2% and most preferablyless than about 1% to about 0.5%, thereby containing from about 95 toabout 99.5% alkylpolyglycoside. The alkylpolyglycoside product nowsubstantially alcohol-free, is removed from the evaporation zone at atemperature of about 390° F., plus or minus about 30° F., about thetemperature employed in the evaporation step. The water employed toprovide the aqueous solution of alkylpolyglycoside for the bleachingprocess of the present invention, may be pre-heated to a temperature ofabout 70° to about 150° F. in a preheater 2 in FIG. 1.

After the process of reaction of the alcohol and saccharide to form thealkylpolyglycoside, the acid is neutralized as discussed earlier above,preferably with an alkali metal hydroxide, such as sodium hydroxide.Most preferably the neutralization is carried out with a mixture ofsodium hydroxide and an alkaline earth metal oxide, such as magnesiumoxide, to a pH level of about 9 to about 11 or 12. The magnesium oxideis employed in an amount effective to provide about 250 to about 1000ppm, or less, of magnesium in the product after evaporation and removalof the alcohol. The aqueous solution of the substantially alcohol-freealkylpolyglycoside and the preheated water will accordingly, preferablycontain less than 1000 ppm magnesium and more preferably about 500 toabout 700 ppm for solution containing about 50% to about 70% d.s.alkylpolyglycosides.

The temperature of the aqueous solution is controlled and maintained ata temperature about the temperature at which the bleaching step will beconducted, and the aqueous solution is then introduced into a vessel orbleaching reactor 3 which is maintained at the bleaching temperature ofabout 85° to about 105° C., preferably about 88° to about 93° C. (about190° to about 200° F.), most preferably at about 88° C. (190° F.).

As shown in FIG. 2, the aqueous solution is introduced into thebleaching reactor 3. In start-up of the continuous process, the reactorwill be filled to a level above the agitator (not shown) in vessel 3before the caustic and bleaching agent (preferably NaOH and H₂ O₂) areintroduced into the reactor vessel. The pH of the aqueous solution isadjusted to a level of about 10 to about 11.5, preferably about 10.2 toabout 10.8, most preferably about 10.3 to about 10.7, (i.e. about 10.5)and maintained at this level during the bleaching process by the use ofalkali metal hydroxide, preferably NaOH as shown in FIG. 2. Thepreferred bleaching agent is hydrogen peroxide, H₂ O₂, shown in FIG. 2.The amounts of NaOH and H₂ O₂ are controlled and metered into the vessel3 in amounts effective to maintain the pH level at the desired level andto effect the reduction in color to the desired level. The hydrogenperoxide preferably in a 35% aqueous solution, will be employed in aweight per weight (wt/wt) dry solids alkylpolyglycoside of about 0.25 toabout 2% wt/wt, more desirably about 0.6 to about 1.25% and mostpreferably at about 1%. The sodium hydroxide will be employed in anamount of about 0.9 to about 1.2 moles NaOH per mol H₂ O₂, preferably atabout 1.1 moles NaOH per mole H₂ O₂.

If magnesium is already present at the appropriate level as a result ofthe use in the neutralization of the acid catalyst after the reactionwherein the alkylpolyglycoside is prepared from the alcohol andsaccharide reaction, no further control thereof is required. Adjustmentof the magnesium level, if necessary, can preferably be made by adding asource of magnesium, such as MgO or MgSO₄ to the aqueous solution priorto introduction to the reactor vessel 3, during preparation of theaqueous solution. However, if convenient to do so, the magnesium oxidecan be introduced directly into the bleaching zone reactor vessel 3. Thelower levels of MgO result in more decomposition of peroxide during thereaction and accordingly to achieve the same color conversion willrequire a higher peroxide dosage and/or higher pH resulting in increasedcost of the operation. Operation above 250 ppm and less than 1000 ppmresults in an optimum balance.

The hydrogen peroxide and sodium hydroxide are typically injectedsimultaneously and the process proceeds with evolution of oxygen fromthe hydrogen peroxide addition, which causes the reaction pressure torise. Foaming occurs in the reactor, but foaming is minimized by notautomatically venting the reactor to maintain atmospheric pressure. Asshown in FIG. 2, a valve 4 which contains a pressure relief valvepermits the pressure in the reactor to rise and be maintained at about40 psig, more preferably about 20 psig. This minimizes foam and permitsefficient bleaching.

The efficiency of the hydrogen peroxide bleaching is influenced by theconcentration of residual peroxide, pH and reaction temperature. At thepH level and temperatures discussed above, residual peroxide levels aremaintained above 200 ppm to about 1500 ppm, preferably below 1000 ppmand most preferably at about 200 to about 400 for greatest efficiency.

In the continuous process, residence time is largely fixed by theproduct rate from the evaporator where the alcohol is removed. Residencetimes on the order of about 5 to about 15 hours are generally used withabout 5 to about 7.0 being preferred at temperatures of about 200° toabout 210° F., most preferably about 6.0 to about 6.5 at thetemperatures of about 205° F. and about 10 to about 12 at temperaturesof about 190° to about 200° F., with about 12 at about 190° F., pH andperoxide levels preferred as discussed above and with feed rates ofsubstantially alcohol-free alkylpolyglycoside melt from the evaporationabove. The temperature is controlled and maintained at the preferredlevel and the feed of caustic and peroxide controlled and maintained toprovide the preferred pH and peroxide levels, at target steady statevalues of about 10.7 to about 11.0 pH and about 200 to about 400 ppmrespectively. These conditions will provide product of the desiredreduced color, which can be monitored throughout the process by samplingand determining the color. The aqueous solution fed to the bleachingvessel will be dark having an extinction coefficient at 470 nm in therange of about 10 to about 15, typically at about 12.5 to about 13. Thetarget steady state value for extinction coefficient of the product atpH of 7 is from about 0.025 to about 0.15, and provides a Klett color of50 or less preferably in the range of about 5 to about 30.

After completion to the desired bleaching level, the product exiting thecontinuous stirred bleach reactor 3 is cooled to about 150° F. (about65° C.) in preparation for completion of the finishing process whichincludes a stabilization treatment, to stabilize the color against anyreversion to a darker color. Such treatment typically involves catalytichydrogenation or treatment with a stabilizing compound, such as alkalimetal borohydrides to further reduce color and stabilize the coloragainst deterioration over long periods of storage. After the finishingprocess, the pH and concentration of the alkylpolyglycoside surfactantis adjusted and placed in storage for sale. In a borohydridestabilization, a peroxide bleached alkylpolyglycoside solution of about50 to about 55% actives concentration, and a pH of about 10 and residualperoxide concentration below about 50 ppm, and preferably below about 25ppm is adjusted to a pH of about 7 with sulfuric acid to eliminate anyhaze, after which the product is adjusted to a pH of about 10 andtreated with a sodium borohydride solution until the borohydrideconcentration is substantially zero.

The invention can best be illustrated by means of the following examplesin which all parts and percentages are by weight unless otherwisespecified. In the preceding description, references have been made tocolor determination expressed as extinction coefficient and as Klettcolor. These color determinations are conducted as set forth in ExamplesA and B below:

EXAMPLE A Extinction Coefficient Color Determination

The extinction coefficient method is a measure of color by absorption.This method uses absorbance at 470 nm, the fraction of dry solids, and`as is` sample weight, diluted sample weight, and densities to arrive atthe extinction coefficient.

The determination is made employing a Spectronic 20, Bausch & Lomb,Spectrophotometer (Catalog No. 33-31-72 or equivalent) and Dispo CultureTubes for measurement, 13×100 mm. (VWR Products, Catalog No. 60825-571or equivalent). The dry solids weight fraction of the as is sample isdetermined. A sample is diluted with a 3/1(v/v) isopropanol/water blendto obtain a clear solution which will give a transmittance between 15%and 85% on the Spectronic 20. The weight of the `as is` sample and ofthe diluted sample is noted and the density (g/ml) of the final dilutionsample is determined. The absorbance of the clear diluted sample at 470nm is measured in the Spectronic 20. The extinction coefficient (ec) iscalculated using the following formula: ##EQU2##

EXAMPLE B Klett Color Determination

In this method the procedure is an empirical measurement of color (broadband absorbance) using a Klett-Summerson Photoelectric Colorimeter,Glass Cell Model 900-3, using a 400-450 nm blue filter No. RS-42 and aKlett cuvette test cell (rectangular, 8×4×2 cm glass cuvetti, Klett partNo. 901, optical path length, 4 cm). After calibration and preparationof the sample cuvette, the absorbance is measured at 5% actives and pHof 7 and the scale reading reported as "Klett color (4 cm )."

EXAMPLE 1

This example is an illustration of a continuous steady-state process ofbleaching of a substantially alcohol-free alkylpolyglycoside meltsubsequent to removal of the alcohol by an evaporation process. SoftenedWater (2900#/hr) was injected into a stream of C₁₂ -C₁₆alkylpolyglucoside melt (3250#/hr). The resulting 6150#/hr blendcontaining 0.23% residual fatty alcohol, was dark in color (extinctioncoefficient @ 470 nm=12.7), and was passed through an in-line staticmixture and through a dip tube into the bottom of a jacketed pressurevessel, equipped with temperature control, an agitator and dip-tubechemical addition lines (50% caustic and 35% peroxide). After about fourhours, sufficient material was present to start the agitator, and thetemperature controller was set at 190° F. and the overflow top exitvalve set to open at 20 psi. Caustic was added at 200#/hr until the pHof the mixture, measured on an "as is" basis at room temperature, wasabove 10.8, at which time the 50% caustic flow was reduced to 77#/hr andthe peroxide flow was started at 90#/hr. These flows were maintainedthroughout the run with minor adjustments to maintain targetsteady-state values of color, pH, residual peroxide of e.c. (@pH=7)=0.05-0.15, pH=10.7-11.0, and residual H₂ O₂ =200-400 ppmrespectively. These targets attained after about 1.25-1.5 reactorvolumes, and the material exiting the continuous stirred bleach reactorwas cooled to about 150° F. in preparation for completion of thefinishing process.

As can be seen from the foregoing description of the related art and thepresent invention, including the examples of the continuous process, ahighly efficient bleaching process is developed which results in anunexpected reduction in color from a dark brown (extinction coefficientof about 10 to about 15 at 470 nm on a 50-55% active aqueous solution)to a very light or white, slightly hazy, solution of an extinctioncoefficient color on the order of about 0.025 to about 0.15. The coloras determined by the Klett method is below about 50 and in the range ofabout 5 to about 30.

We claim:
 1. A method of reducing the color of an alkylpolyglycosidecomprising the steps of(a) providing an aqueous solution of thealkylpolyglycoside; (b) continuously introducing the aqueous solutionfrom step (a) to a bleaching zone maintained at a temperature of about85° to about 105° C.; (c) adjusting and continuously maintaining the pHof the aqueous solution in said bleaching zone at a pH of about 10 toabout 11.5; (d) contacting the aqueous solution with a peroxy bleachingagent in an amount effective to bleach and reduce the color of thealkylpolyglycoside and in the presence of more than about 250 ppm andless than about 1000 ppm Magnesium (Mg); and (e) continuously removingalkylpolyglycoside from said bleaching zone, wherein thealkylpolyglycoside has a Klett color below about 50, an extinctioncoefficient color from about 0.025 to about 0.15 and a residualbleaching agent level below about 1000 ppm.
 2. A method as defined inclaim 1, wherein the alkylpolyglycoside in step (a) is the reactionproduct of an alcohol and a saccharide in the presence of an acidcatalyst having an extinction coefficient color of about 10 to about 15.3. A method as defined in claim 2, wherein said alkylpolyglycoside instep (a) contains less than about 5% by weight of the alcohol from thereaction of the alcohol and saccharide.
 4. A method as defined in claim3, wherein the alkylpolyglycoside in step (a) contains less than about1% by weight of the alcohol.
 5. A method as defined in claim 3, whereinsaid aqueous solution in step (a) contains about 30% to about 85% byweight alkylglycoside.
 6. A method as defined in claim 5 wherein saidaqueous solution contains about 50 to about 75% alkylglycoside.
 7. Amethod as defined in claim 6 wherein said aqueous solution containsabout 55% by weight alkylglycoside.
 8. A method as defined in claim 1wherein the temperatures in said bleaching zone is maintained at atemperature of about 88° to about 93° C.
 9. A method as defined in claim8 wherein the temperature is maintained at about 88° C.
 10. A method asdefined in claim 1, wherein the pH of the aqueous solution in thebleaching zone is maintained at about 10.2 to about 10.8.
 11. A methodas defined in claim 10 wherein the pH is maintained at about 10.5.
 12. Amethod as defined in claim 10 wherein the aqueous solution in thebleaching zone contains Mg in the form of MgO in an amount of about 250to about 1000 ppm.
 13. A method as defined in claim 1 wherein thebleaching agent is hydrogen peroxide.
 14. A method as defined in claim13, wherein the pH is maintained in the bleaching zone with sodiumhydroxide.
 15. A method as defined in claim 14, wherein the hydrogenperoxide is employed in an amount on a weight per weight basis ofperoxide to dry solids alkylglycoside of about 0.25 to about 2% and thesodium hydroxide is employed in an amount of about 0.9 to about 1.2moles of sodium hydroxide per mole of hydrogen peroxide.
 16. A method asdefined in claim 15 wherein the hydrogen peroxide is employed at about1% and the sodium hydroxide is employed at about 1.1 moles sodiumhydroxide per mole of hydrogen peroxide.
 17. A method as defined inclaim 15 wherein the aqueous solution in the bleaching zone contains Mgin the form of MgO in an amount of about 250 to about 1000 ppm.
 18. Amethod as defined in claim 1 wherein a pressure is maintained in thebleaching zone up to about 40 psig and the residence time of thealkylglycoside in the bleaching zone is about 5 to about 15 hours.
 19. Amethod as defined in claim 18 wherein the pressure is maintained in thebleaching zone at about 20 psig and the residence time is about 6 toabout 6.5 hours.
 20. A method as defined in claim 1 wherein(1) thealkylglycoside in step (a) has an extinction coefficient color of about10 to about 15, contains less than about 5% of the alcohol from whichthe alkylglycoside was prepared, and is contained in the aqueoussolution of step (a) in an amount of about 30 to about 85% by weight;(2) the temperature in the bleaching zone is maintained at about 88° toabout 93° C.; (3) the pH of the aqueous solution in the bleaching zoneis maintained at about 10.2 to about 10.8 with sodium hydroxide; (4) thebleaching agent is hydrogen peroxide employed in an amount on a weightper weight basis of peroxide to dry solids alkylglycoside of about 0.25to about 2% and the sodium hydroxide in (3) is employed in an amount ofabout 0.9 to about 1.2 moles sodium hydroxide per mole of hydrogenperoxide; (5) the aqueous solution in the bleaching zone contains Mg inthe form of MgO in an amount of about 250 to about 1000 ppm. (6)pressure is maintained in the bleaching zone up to about 40 psig and theresidence time in the bleaching zone is about 10 to about 15 hours; (7)the alkylglucoside removed in step (e) has an extinction coefficientcolor from about 0.25 to about 0.15.
 21. A method as defined in claim20, wherein the alcohol in (1) is less than about 1% by weight and theaqueous solution in (1) contains about 55% by weight alkylglycoside; thetemperature is maintained at about 88° C.; the pressure is maintained atabout 20 psig; the residence time is about 12 hours; the hydrogenperoxide is employed in an amount of about 1% weight per weight basis ofperoxide to alkylglycoside and the sodium hydroxide is employed in anamount of about 1.1 moles per mole of hydrogen peroxide; and the pH ismaintained at about 10.5.
 22. An alkylglycoside having a color reducedfrom an extinction coefficient color of about 10 to about 15 to anextinction coefficient color of about 0.025 to about 0.15.